This invention relates generally to irrigation control systems, and, more particularly, to digital systems for the control of a large number of irrigation or sprinkler valves in a desired sequence.
Large sprinkler systems, whether used to irrigate agricultural property, sports facilities, or other types of property, all include a large number of valves, which are typically solenoid-operated, and which must be opened and closed in a desired sequence of operations as determined by the terrain, climate and other factors. For very large systems, it is both costly and impractical to run a pair of wires from each of the valves to back to a central controlling device. Accordingly, it is a principal objective in designing such systems to reduce the number of conductors between the central controller and the valve locations to an absolute minimum, i.e., ideally to two wires. Inherently, this objective dictates the use of some form of control signal encoder at the central site and a control signal decoder at each valve location.
Control systems utilizing a central encoder and a number of remote decoders connected to the encoder by only two wires, or by one wire and a ground return, are not unknown. However, such systems have heretofore utilized a technique in which device addresses and control signals are encoded in the form of relatively high-frequency signal bursts, and are then decoded by appropriate filtering at each of the decoders. For example, U.S. Pat. No. 3,821,559, issued in the names of Ueda et al., discloses such a system for the digital control of a number of electrical devices in an automobile.
The present invention is concerned with improvements in a digital two-wire control system of which the basic principles have been disclosed in a publication by the inventor. In accordance with these principles, power is transmitted from a central controller and encoder to a number of decoders over two wires and in the form of an alternating current. Control information in the form of a decoder address and an on/off code is encoded into the alternating-current signal by selectively suppressing current flow in one direction to indicate binary values of the digits transmitted as control information. At each decoder, the control information is decoded, and the alternating-current signal is rectified to provide power to operate valves under the control of the decoded information.
In his originally published description of the basic invention, the inventor proposed that blocks of control information be separated by strings of all zeros, i.e., by cycles of the alternating-current signal in which one half of each cycle had been clipped to zero, and also proposed that the encoder and decoder logic be implemented using standard transistor-transistor logic (TTL). However, the use of strings of zeros to separate blocks of control information has the significant disadvantage that the power-carrying capacity of the lines from the encoder is substantially reduced. Furthermore, the use of TTL logic imposes the need for a highly stable power supply, and, more importantly, because of its relatively high power consumption, is not suitable for relatively large systems having many decoders and valves. It is to these particular problems that the improvements of the present invention are directed.